1. Field of the Invention.
The invention relates generally to thermal treatment of silicon wafers. In particular, the invention relates to thermal treatment of silicon wafers to be used to make photovoltaic cells.
2. Description of the Related Art.
There is currently great interest and intense development in photovoltaic (PV) solar cells which directly convert solar radiation to electrical power. Although many different configurations and even different semiconductor materials have been proposed, an efficient and economical solar cell can be fabricated using structures, materials, and techniques long known for silicon integrated circuits. Specifically, a single crystal or multi-crystalline silicon wafer is processed to have a large-area p-n junction extending across a substantial fraction of the wafer. One class of silicon PV cell is made on wafers cut from an ingot of monocrystalline silicon grown by the Czochralski (CZ) process in which a monocrystalline silicon seed crystal is used to start the freezing of silicon by contacting silicon melt held at about 1420° C. and the silicon ingot is drawn from the melt. The melt is contained in a heated crucible conventionally composed of fused quartz (silicon dioxide). A single silicon ingot intended for integrated circuits or PV applications is typically drawn from a crucible which is discarded after a single ingot. On the other hand, solar applications may benefit economically from a so-called continuous Czochralski process in which multiple ingots are drawn from a crucible which is recharged with silicon during the drawing process.
Silicon wafers usable for solar cells consist of elemental silicon and less than a total of 1 at % of dopants, oxygen, and other impurities. All monocrystalline silicon produced by the CZ process contains low but significant levels of oxygen in the ingot, typically at concentrations of about 1×1018 cm−3 or 20 ppma (parts per million atomic) relative to the silicon. The oxygen originates primarily from the quartz crucible. At the growth temperature, most of the oxygen is interstitial in the lattice, that is, single oxygen atoms residing in the interstices of the crystalline silicon lattice. However, some of the oxygen agglomerates together in various atomic configurations, also called cluster or oxygen precipitate nuclei. The precipitate nucleation occurs because, after crystallization at the silicon melting point of around 1420° C., as the crystal cools as the growing ingot is drawn away from the melt, oxygen becomes supersaturated in the silicon lattice. As a result, the free energy of the system is lowered by oxygen clustering as the ingot cools. Some of the oxygen coalesces around several silicon atoms to form small clusters called nuclei of size between about 6 and 35 nm. More nuclei can form if the crystal is post heated to between 650 and 850° C. Very rapid heating will dissolve the nucleated oxygen nuclei. However, in the as grown ingot, a long anneal above about 1000° C. will cause the oxygen to form much larger precipitates up to a few microns in size. Rapid heating cannot dissolve these large precipitates.
The initial clusters and nuclei are referred to as “grown in” defects. Both nuclei and precipitates are considered to be defects in the lattice which act as recombination centers or traps for electrons and holes in the crystalline solid, thereby decreasing the critical parameter for high-efficiency solar cells of minority carrier recombination lifetime. Removal or elimination of these oxygen defects increases the minority carrier lifetime (mel) in the silicon. Oxygen may further degrade an operating boron-doped silicon PV cell by causing a photo-induced degradation in which the solar irradiation activates the formation of boron-oxygen scattering in the silicon.
For the past thirty years, the effects of oxygen in CZ silicon for integrated circuit (IC) applications have been extensively studied. During CZ growth, oxygen in the silicon melt is incorporated into the growing silicon ingot typically in isolated interstitial locations in the silicon crystalline lattice at concentrations stable at the high temperatures. However, as the ingot gradually cools as it is slowly drawn from the melt, the equilibrium oxygen solubility limit decreases and oxygen incorporated at the interface during growth becomes super saturated. Assuming a moderately high temperature above for example 1200° C., the oxygen can diffuse through the silicon lattice to form a lower-energy state of oxygen precipitates believed to be regions of a few oxygen atoms in associated or chemically bonded with silicon in the lattice. Such oxygen precipitates act as defects which strain the silicon lattice and therefore act as gettering centers for metallic impurities during subsequent heat treatment typically experienced during semiconductor device fabrication. Except where those precipitates are near the surface, they are usually not important for integrated circuit applications. However, these oxygen precipitates (also referred to as gettering sites) throughout the entire wafer dramatically lower the minority carrier recombination lifetime of silicon, which is critically important for solar cells.
It has been observed that silicon with an oxygen concentration of less than 10 to 13 ppma (parts per million atomic, as measured according to the ASTM (American Society for Testing and Materials) standard for measuring oxygen in silicon) will not experience significant oxygen precipitation during device fabrication. Single crystal silicon can be also produced formed by the float zone (FZ) process, which does not use a quartz crucible. The FZ process produces silicon containing little if any oxygen, for example, less than 1 ppma oxygen dissolved in the silicon lattice. FZ silicon exhibits very high minority carrier lifetimes arising from the absence of oxygen defects and it thus enables high efficiency PV cells to be produced. However, FZ silicon is prohibitively expensive and is not used in commercial-scale terrestrial solar cell applications.
For CZ silicon, various techniques are applied to reduce the oxygen concentration in the ingot to below 13 ppma but the techniques introduce operational constraints and have not been completely successful.
Most modern IC's have active semiconductor devices formed in a surface region within about a few microns of the original wafer surface. For all IC types, low oxygen precipitation in the active surface region is not desired. On the other hand, for high yield in IC fabrication, controlled oxygen precipitation in the central plane of the wafer is desired because the precipitates not only “getter” metallic impurities but also increase the strength of the silicon substrate during the many high-temperature processing steps required for advanced IC's. Controlled oxygen nucleation and precipitation away from the surface of the silicon wafer help pin dislocations due to process induced thermal stress and thus reduce the tendency for crystallographic slip and wafer warping without interfering with the active devices which are usually within the first 20 microns from the surface. In addition to imparting strength, oxygen is desired during IC processing because the lattice strain caused by precipitates getters metals towards the precipitates and away from the active devices, thereby improving yield and performance of the IC's. Further, because almost all the IC's made today are majority carrier devices, such as MOSFET's, minority carrier lifetime is not considered to be very important. In fact, metals are often deliberately introduced by ion implantation to “kill” lifetime in specific regions around the IC devices. For these reasons, a “denuded zone” process has been developed for reducing oxygen precipitation in the surface layer but leaving some precipitates in the wafer interior. Falster describes the denuded zone process in the background section of U.S. Pat. No. 6,336,968, incorporated herein by reference in its entirety.
Falster further describes, what is sometimes called, a tabula rasa process to avoid oxygen precipitation when the wafer is used in the fabrication of an electronic integrated circuit such as a DRAM. It is important in IC's not only to reduce the precipitates but also to prevent their reformation in the surface region during subsequent hot processing such as a long anneal at a temperature of 1000° C. In the Falster process performed before IC processing, the silicon wafer is subjected to a fast anneal in a rapid thermal processing (RTP) chamber at a temperature of greater than 1150° C. in an oxygen ambient. Falster believes that the high-temperature oxygen anneal reduces vacancies and thus prevents precipitation near the surface region during high-temperature processing used for IC's.
The Falster process is believed to prevent oxygen precipitation near the surface regions and thus does not enjoy the advantages of interior oxygen precipitation described above for the denuded zone process.